1. Field of the Invention
The invention is directed to a printing device for the production of automatically readable script on documents by means of a spoke type wheel, a stop mechanism for the documents, and a transporting mechanism, comprising a step motor with micro-step control for the document for producing the character spacings. It is also directed to a printing device for the production of automatically readable script on documents by means of a spoke type wheel, a stop mechanism for the latter and a step motor with micro-step control for the adjustment of the spoke type wheel in the correct printing position.
2. Description of Related Art
The article by Hans Gugg and Herbert Sax: "Schrittmotoren - optimal angesteuert", ELEKTRONIK 1980, issue 26, pages 43-49, provides suggestions as to how step motors can be controlled by means of so-called micro-step control in such a way that they not only move in individual large steps, but, also, each individual step may be divided into a plurality of micro-steps, wherein it is possible to carry out the arrangement in such a way that each individual step is divided into two, four or six micro-steps. This is achieved by means of winding currents which taken an approximately sine-shaped course through the above-mentioned micro-step control, wherein it is possible, however, to keep the current in the winding in such a way that there results a corresponding stopping point for the part driven by the step motor. The step motors, which are controlled by means of a micro-step control, thus operate substantially like a standard step motor which moves its load in individual steps, the difference being that each individual step is additionally divided into a plurality of micro-steps, wherein the number of micro-steps is selectable corresponding to the design of the circuit.
According to the above article, the following description contains a proposal for a circuit for the control of a 2-page motor with continuous phase current regulation which is realized with the interface ICL 291 and the output ICL 292.
The premise for the linear modulation of the magnetic field of the step motor is the accurate control of the phase currents of its windings. FIG. 7A shows the basic circuit of a bipolar control, to which this discussion will be limited. Two modes of operation can be utilized for the depicted solution:
a) analog current sources; PA0 b) switched current sources. PA0 supply voltage maximum 36 volts; PA0 output current .+-.2 amps.; PA0 zero symmetrical and positive control input; PA0 transmission factor I.sub.out /U.sub.in externally programmable; PA0 four quadrant operation with energy feedback; PA0 high efficiency.
Analog solutions are sufficiently known with their advantages and disadvantages. Their application range in this mode of operation, is limited by the resulting output losses. Switching solutions are, in addition to their higher efficiency, particularly advantageous if the load is simultaneously utilizable as a storage inductance. Thus, external coils are no longer required which applies in the case of the step motor.
The integrated circuit converts the zero symmetrical input control voltage U.sub.in into a pulse width modulated signal by means of an internally generated triangle, which controls the output portion (FIG. 7B). In order to control the load current I.sub.m, said current is converted by the resistance R.sub.S into a momentary value, which serves as negative feedback information GK.
The most important data for this integrated circuit are as follows:
The control of a 2-phase step motor according to the above-mentioned principle requires two sine functions with 90.degree. shift. They can be produced by different circuit technology solution types: First at all by an analog sine-cosine function generator and then by a purely digital signal preparation with D/A Converters. The advantage of the last-named principle, with which the following description deals, lies, among other things, in the simple realizability of the micro-step.
FIG. 7C shows the functional mode of the D/A converter L 291. It contains five bi-directional constant current sources of a value equal to 2.sup.0 . . . 2.sup.4, said current sources being switchable by control bits. There are two additional possibilities of affecting or modulating these current sources:
1. Their common current direction is to be fixed through the logic input SIGN.
2. The magnitude of the input current DACIN determines its absolute value.
The resulting current thus produced can be tapped at the output DACOUT--with the internal operational amplifier V.sub.1 it is converted into an equivalent voltage, which is compatible to the output current source as far as level is concerned. The current as well as the voltage output are 0-symmetrical, meaning they can be of positive or negative magnitude.
The signal generation with hardwired logic is indeed a basic possibility, but is, however, mostly rejected because of the high cost and the insufficient flexibility. In today's state of the art, the microcomputer is the most sensible device for this purpose. With this help, the required signal shape sequences can be turned by means of software optimally onto the step motor system.
The interaction of the individual components is discernible from FIG. 7D. This Figure also contains a detailed resolution of the block diagram of the output current sources L 292. By means of the regulator R, which acts on both D/A converters, the output amplitude and thus, the peak current of the step motor phases is adjusted to the motor type utilized. The dimensioning of the external negative feedback network of the output current source L 292 has an influence on the step function of the output current--it must therefore be individual adapted to the motor impedance. If one now connects the comparator inputs (terminal 4) of the end steps, whereby an oscillator network can be eliminated, then both of them work with the same timing frequency. For a control output of 60 watts per step motor phase, the space requirement of the overall structure of the end step is very small comprising 63 cm.sup.2.
In step motors the influencing of the effective levels of the phase current as a function of the momentary operational situation is often desirable.
In order to achieve short acceleration and braking times, a high torque is required. This, however, can only be achieved by high phase currents, which simultaneously drive up the motor performance losses and thus, the temperature. Therefore, one must, in case one does not change the current, find a compromise between torque and motor heating. Higher phase currents are allowable for short periods (they are limited by winding current density and danger of demagnetization of the rotor), if, for the remaining time in which the motor is inoperative, the current is reduced. The positioning effect diminishes proportionally to the current, which, however, is only of importance if, in addition to the mass inertia of the load, high frictional forces are present. If the possibility exists to influence the effective motor current in addition to the control timing, then one achieves shorter acceleration and braking times with simultaneously improved efficiency of the overall system.
The variation of the effective motor current is possible in a simple manner in the circuit concept in FIG. 7D, by the D/A converter L 291, the output signal amplitude can be varied by the magnitude of a current flowing into the terminal 9.
FIG. 7E, shows a solution with a logic controllable operational amplifier configuration which is not used here (V21) which is additionally located on the chip. The amplifiers of the two integrated circuits are switched through a resistance matrix as 2-bit-D/A converters, which can adjust the motor effective current through a microcomputer in four stages (FIG. 7F).
In order to achieve a constant angular velocity in the range of low step frequencies within a full step, the digitally produced control curve must have as fine as resolution as possible. This means that the timing frequency is n-times higher compared to the full step frequency. The sensible limits of the resolution of the motor current are set by the integration behavior of the inductance. If this high resolution remains further present during increase of the step frequency, the experience has shown the output speed of the microcomputer limits the step frequency of the motor, which would still maintain its functioning ability with considerably higher time rates.
As already previously stated, the control curve shape increasingly loses its significance with increasing rpm, this is because of the current integration of the motor. A coarsening of the quantification therefore does not have disadvantageous effects at higher rpm's and, is thus, a legitimate means to increase the upper limit frequency of the system up to 2n-times. The timing situation occurring thereupon is comparable with the conventional rectangular control.
It is a particularity that the microcomputer can stop the rotor of the step motor in any position within a full step. The accuracy of the intermediate positioning is determined by the linearity of the magnetic field of the motor, its retention movement and the quantification factor of the current. A half step - or quarter step positioning is realizable without difficulty in most cases without these limiting factors. This fact opens the possibility to utilize coarse step motors in systems, in which a micro-step angle is required. Herein two advantages result:
1. With a constant timing rate situation, the speed of the motor increases, because it covers a larger angle of rotation per step.
2. The cost of these motors is mostly lower compared to those having higher resolution, wherein the overall costs can be reduced.
The possibility of even higher angular resolutions beneath the quarter step exists if angle encoders are used, by means of which a positioning error can be corrected through a regulation loop.
Preparation of the sine-wave current curve, reduction of the current resolution as a function of speed, micro-step positioning, and phase current control depending on acceleration are tasks and problems which are to be solved by the microcomputer.
When printing automatically readable documents, there are sometimes problems with script as, e.g. the so-called E 13B, because in this case, according to convention, the characters are to be printed in the print area provided for them, not in the center, but, rather, flush right, for example. When this kind of script is arranged on the individual spokes of a spoke type wheel then, during standard use of a step motor, there follows the inevitable consequence that the characters must also be arranged on the spokes so as to be off-centered in order to arrive in the correct position when printing. As a result of this arrangement, the spokes which carry this type twist during printing because of the characters which are arranged to one side. As such, a premature breakage of the corresponding type spoke cannot be ruled out. Moreover, in such an arrangement of the type to one side of the spoke, for example, the type 1, a clean printing of the type can not be ensured to the same extent as when the type is symmetrical, as, for example, the type of the numeral 8.